This invention relates generally to electric lamps and particularly to metallic halide lamps for use, for example, on motor vehicles or the like. More particularly, the invention deals with improvements concerning the molybdenum foils that are embedded in the pinch seals of a metallic halide lamp for electrically connecting the pair of discharge electrodes to leads.
Generally, a metallic halide lamp has its envelope composed of a length of glass tube, with its opposite ends pinch sealed to provide a hermetically closed discharge chamber therebetween in which there are contained substances such as a metallic halide, xenon, and mercury. A pair of electrodes are partly disposed in the discharge chamber and partly embedded in the pinch seals. Also partly embedded in the pinch seals, a pair of leads are connected to the electrodes via molybdenum foils to which the instant invention pertains.
There might be contemplated the use of either hard glass or silica glass for the lamp envelope. For its higher coefficient of linear expansion, 50.times.10.sup.-7 (1/.degree. C.), hard glass is undesirable because the envelope of a metallic halide lamp is heated to a temperature of as high as 900.degree. C. in use, at which hard glass would be deformed or lose transparency. As an additional disadvantage, hard glass contains substances in addition to silica. These additives would luminesce on arc discharge, imparting an undesired hue to the color in which the lamp is intended to give off light.
Silica glass is therefore a currently preferred material for the envelope of a metallic halide lamp. However, silica glass demands some special considerations in its use for high temperature applications, as in this one, by reason of its low coefficient of linear expansion, 5.5.times.10.sup.-7 (1/.degree. C.). Usually, the lead wires of a metallic halide lamp are made from molybdenum, and the electrodes from tungsten. Molybdenum lead wires are 60.times.10.sup.-7 (1/.degree. C.), and tungsten electrodes 40.times.10.sup.-7 (1/.degree. C.) , in coefficient of linear expansion. It is therefore undesirable to bury any extended lengths of the molybdenum lead wires and the tungsten electrodes in the pinch seals by directly interconnecting them.
This difficulty has been overcome, though conventionally to a limited extent, by interposing foils of molybdenum between electrodes and leads and burying the molybdenum foils in the pinch seals. The molybdenum foils are less affected by the difference in coefficient of linear expansion despite the expansion and contraction of the glass envelope which are repeated numerous times in use of the lamp. Less relative displacement and less gap creation are known to occur between molybdenum foils and pinch seals than between molybdenum wires and pinch seals, and so are less cracks and consequent leakage of the gasses from the discharge chamber.
The foregoing evaluation of the conventional molybdenum foils does not imply that they are per se satisfactory; only, they provide better results than if the tungsten electrodes were directly connected to the molybdenum lead wires within the pinch seals. A difference in coefficient of thermal expansion does exist between molybdenum foils of conventional make and silica glass pinch seals. Relative displacement and gap creation do occur between them, and so do cracks and gas leakage. These shortcomings have made the useful life of prior art metal halide lamps less than a half of their expected lifetime.
So the present applicant has conducted extensive microscopic studies of the prior art molybdenum foils within the pinch seals of metallic halide lamps in order to pinpoint the reasons for the insufficient service life of conventional metal halide lamps. The studies have revealed the following findings:
1. The recrystallization grain size of the prior art molybdenum foils is generally over 50 microns; more particularly, from 50 to 100 microns in the case of 99.95% pure molybdenum, and 100 microns or more in the case of 99.90% pure molybdenum. PA1 2. The prior art molybdenum foils provide a smooth interface with the silica glass pinch seals as viewed on the level of recrystallization grains.
The term "recrystallization grains" is used in the foregoing findings because the crystal grains of the prior art molybdenum foils within the pinch seals of metallic halide lamps are obviously unlike those before the foils are embedded in the pinch seals. Manufactured by rolling, as is usually the case, molybdenum foils have their crystal grains rendered as fine as from five to thirty microns. The foils undergo recrystallization, however, into larger crystal grains on being heated and compressed via the heated glass when the glass envelop is pinch sealed at temperatures of 1600-2000.degree. C., which are far above the softening point of silica glass. Molybdenum recrystallization is known to start at about 1300.degree. C. Subjected to the much higher temperatures of 1600-2000.degree. C. and pressurized at the same time, the prior art molybdenum foils within the pinch seals have readily recrystallized into much coarser grains of over fifty microns.
Having been embedded as above in the pinch seals with their crystal grains made inconveniently larger in size, the prior art molybdenum foils have been easy to expand on being heated each time the lamp glows. The molybdenum foils on thermal expansion have exerted correspondingly great stresses on the pinch seals until slips occur at the interface between molybdenum and glass. Such slips and resulting gaps have grown greater as the lamp is repeatedly switched on and off in use, with the accompanying expansion and contraction of the molybdenum foils and the glass pinch seals at different rates. Furthermore, when the lamp is glowing, the gas pressure in the discharge chamber rises, conventionally finding its way into the gaps between glass and molybdenum of the pinch seals via gaps that are produced around the electrodes in a like manner. Accelerated by these migrant gas pressures, the gaps have eventually grown into cracks permitting gas leakage from the discharge chamber into the atmosphere. Hence the unwarrantably short service life of metallic halide lamps with the conventional molybdenum foils of large recrystallization grains.